This invention relates to an ink-jet printing device, in which printing is carried out by causing ink particles to be selectively emitted by a nozzle. Various types of selective ink-jet printers are known. In one known type of printer, the liquid ink droplets are generated by causing a selective increase in the nozzle pressure, for example by piezoelectric means. It has also been proposed to use a conductive liquid ink and to eject the ink particles electrostatically, by creating a very large voltage of several thousand volts between the ink and an electrode disposed in front of or behind the paper.
In both these types of printing device, the droplet generally assumes a spherical shape with a diameter of several microns, which becomes deposited on the paper.
Especially in the case of high definition printing, several droplets have to be generated which become partially superimposed on the paper, and it is therefore necessary to be able to dry the mark quickly. For this purpose it has been proposed to use porous paper because of which on the one hand the mark appears blurred, and on the other hand the advantage of being able to print on any paper is lost. It has also been proposed to dry the printed mark by heating or by a cold air jet, which makes the device complicated and costly. A further serious drawback of such printing devices derives from the fact that the liquid ink in the nozzle tends to dry and thus form incrustations. These not only make it difficult to commence printing after a certain period of inactivity, but even during apparently continuous printing they cause the droplets to assume dimensions which vary according to the inevitably variable time interval between one droplet and the next.
There is also known an ink jet printing device wherein the liquid ink is kept at a predetermined level in a small tube having its free end directed upwards. Inserted in the tube are two electrodes located on the same horizontal plane, whereby they remain submerged under an ink layer of a predetermined thickness. The ink jet is generated by an instantaneous vaporization of the portion of ink inside the tube located between the two electrodes, so as to expel the ink layer upwards.
Particularly, in a first embodiment of this printing device, the ink is electrically non-conductive and the vaporization is produced by the dielectric breakdown of the ink, thus producing a spark between the electrodes.
In another embodiment of the printing device, the ink is electrically conductive but has a rather high electric resistivity. The ink is contained in a reservoir, where it is preheated at a temperature slightly lower than the boiling point of the ink. By exciting the two electrodes with a voltage pulse, a current flows into the ink and therefore a great deal of heat is instantaneously produced, thus vaporizing a portion of ink between the electrodes and expelling the ink layer upwards.
Both embodiments of this printing device having two submerged electrodes have the disadvantages of requiring a tube of relatively large diameter in order to house the electrodes, whereby it is not possible to print small enough dots for a high definition printer.